[libcxx][test] Use python specified by build rather than system default python

As of e9564c36, libcxx/gdb/gdb_pretty_printer_test.sh.cpp
fails locally for me because the REQUIRES check for host-has-gdb-with-python
uses python, which for me expands to python 2.7.18. This failure does not seem
to be caught on any upstream builders, potentially because they don't have gdb,
python, or a version of python that makes the test UNSUPPORTED (like python3).

This updates the check to use the python specified by the build (which should
be the python that runs this code), rather than just python.

Differential Revision: https://reviews.llvm.org/D110887